Theses

In our every day life we are surrounded by materials composed of the elements of the periodic table. Rarely does one ask where these elements came from. It has been a long process of discovery to understand the precise origin of many of the elements we consider commonplace. It is now believed that the Big-Bang produced only the lightest elements, primarily hydrogen and helium, and that heavier elements were synthesized as the product of nuclear reactions within stars. Occasionally the nuclear reactions that occur within stars produce an isotope of an element which is unstable, radioactive. When a radioactive species decays it emits radiation which is characteristic of the species that decayed. Satellites have been able to detect the characteristic radiation from the decay of several isotopes in the Milky Way. One in particular which has been detected is the isotope of titanium, ⁴⁴Ti. The decay of ⁴⁴Ti has been seen in the ashes of exploding stars, vast gas clouds termed supernova remnants. This isotope of titanium eventually decays to a stable isotope of calcium found everywhere on Earth from bones to chalk. It is believed that the bulk of the production in stars of ⁴⁴Ti occurs as the star explodes, during the supernova. Calculations indicate that among the many possible reactions during a supernova, a particular nuclear reaction, where calcium captures a helium nucleus and fuses into titanium, is the main source of ⁴⁴Ti. In this work it is detailed how using laboratory equipment on Earth one is able to shed light on the nuclear physics of this particular reaction governing the production of an isotope in our universe.

cvothesis.pdf

Author: Ouellet, C.

Journal:

In the beta-decay of the ground state of ²⁶Al (denoted ^26gAl, t1/2 = 7.2 x 10⁵ y), a characteristic 1.809 MeV gamma-ray is emitted. This signature of the presence of ^26gAl has been widely observed throughout the Galaxy. Indeed, the observation of this gamma-ray proves the ongoing nucleosynthesis of ^26gAl in astrophysical environments, given its short half-life on cosmological timescales. Reproduction of the Galactic ^26gAl steady-state abundance implied by the observations (~ 3 M␣) provides a powerful constraint on nucleosynthesis model calculations. These calculations may also be used to determine the relative contributions to the ^26gAl abundance by different types of astrophysical phenomena.

The amount of ^26gAl produced in nova explosions on oxygen-neon-magnesium white dwarfs is thought to be relatively minor (~ 0.1 - 0.4 M␣). Nuclear uncertainties in the ²⁵Al(p,γ)²⁶Si and ^26gAl(p,γ)²⁷Si reactions may change this by a factor of ~2, however. A direct study of the ²⁵Al(p,γ)²⁶Si reaction has been proposed and accepted at the TRIUMF- ISAC radioactive beams facility in Vancouver, Canada, and is awaiting the production of a sufficiently-intense ²⁵Al beam. To both guide this direct study, and to improve the accuracy of the current ²⁵Al(p,γ)²⁶Si calculations (based on indirect measurements), we have made a new measurement of the ²⁶Si mass. We find the mass excess of ²⁶Si to be∆(²⁶Si) = -7139.5 ± 1.0 keV; this new mass leads to a reduction in the ²⁵Al(p,γ)²⁶Si rate by as much as ~30% at nova temperatures. We have also made new measurements of the energy and strength of a key resonance for the ^26gAl(p,γ)²⁷Si reaction: we find ERCM = 184 ± 1 keV and ωγ = 35 ± 4stat ± 5sys μeV. These results lead to a decrease in the ^26gAl(p,γ)²⁷Si rate by as much as ~15% at nova temperatures.

Our measurements of the ²⁶Si mass and the resonance in ^26gAl(p,γ)²⁷Si both imply an increase in the ^26gAl yield from novae, but still confirm the secondary nature of their contribution to the Galactic abundance of ^26gAl.

anuj_thesis.pdf

Author: Parikh, A. R.

Journal:

A gamma ray array to detect the characteristic gammas emitted from astrophysically significant, radiative proton and aplpha capture reactions, was built as part of the Detector of Recoils And Gammas Of Nuclear reactions (DRAGON) spectrometer at ISAC/TRIUMF. The DRAGON array consists of a collection of 30 hexagonal BGO detectors measuring 7.62 cm long by 5.58 cm across the face. Experiments at DRAGON are affected by background due to "leaky beam" which reaches the end detector along with the reaction products of interest. In many cases the cross sections of these reactions are so small that it is impossible to distinguish the reaction recoils from leaky beam by using only the electromagnetic separator (EMS) of DRAGON. Further suppression of leaky beam is achieved by demanding a time of coincidence between reaction recoils and the associated gamma emitted from the reaction. To determine the rate of gamma/recoil ion coincidence events it is necessary to have an accutate estimate of the gamma array efficiency. Since it is impossible to measure this rate for all experimental conditions it is necessary to have a simulation which can estimate the efficiency of the array for a given set of experimental parameters (e.g gamma energy). A simulation was built with the particle-tracking program GEANT v3.21. The efficiency of the array was measured using calibrated sources of various gamma energies and compared to simulated results. For the cases where the activity of the source was not well know the sources were calibrated using a standard NaI detector of known efficiency. The agreement between simulation and measured differences is more than adequate for proposed DRAGON experiments. The analysis and results of the comparison between measured and simulated efficiency will be discussed in this thesis.

dariothesis.pdf

Author: Gigliotti, D. G.

Journal:

The DRAGON facility at TRIUMF-ISAC was designed to measure the rates of astrophysically important nuclear reactions involving radioactive reactants. To this end, the mass spectrometer was designed to separate the result of a radiative proton or alpha capture reaction, between beam and target nuclei, from the beam itself. Yields are typically on the order 10⁻⁹ to 10⁻¹⁵, thus, the feasibility of a particular reaction is driven by the suppression of the relatively intense beam, to that of the capture product. In the case of Nova explosions, important resonances occur at low beam energies (0.15 to 1.0 MeV/u) where the DRAGON suppression may be reduced.

An MCP (Micro Channel Plate) detection system has been commissioned to be used in a local time-of-flight approach for particle identifcation at the focal plane of the DRAGON recoil mass separator. It is the goal of this additional detection system to enhance the current suppression systems without signifcant loss in effciency. Three properties of the MCP system have been investigated: the timing resolution, the effciency and the position resolution. Two sources, ⁶⁸Ge and ¹⁴⁸Gd, were used off-line to test the detection system performance. The timing studies were performed with the use of a fast PMT (Photo Multiplier Tube) as a second detection system. A DSSSD (Double Sided Silicon Strip Detector) was used for the effciency tests and masks were used during the position resolution studies. These off-line tests were followed by on-line studies of the well known resonance (Ecm = 258.6 keV) in the ²¹Ne(p,γ)²²Na reaction. A simulation using the RELAX3D software along with a custom made tracking code, both written at TRIUMF, has also been studied, and its results pertaining to the three aforementioned important properties will be discussed.

lamey_thesis.pdf

Author: Lamey, M. J.

Journal:

The ¹³N(p,γ)¹⁴O reaction is important as it determines the breakout from the CNO cycle to the HCNO cycle. Studying the ¹³C(ρ,γ)¹⁴N reaction was important for the DRAGON facility at TRIUMF for their future analysis of the ¹³N(p,γ)¹⁴O reaction, not only because pure radioactive ion beams of 13N are impossible to create without contamination from ¹³C due to the very small mass difference between these two elements, but also it was a good test for the DRAGON due to the fact that the ¹³C(p,γ)¹⁴N reaction has been measured before.

Early analysis of the ¹³C(p,γ)¹⁴N reaction data collected by DRAGON, showed that not all the ¹⁴N recoils made it through the DRAGON separator to the end detector (an ionization chamber), because they were being clipped due to the large cone angle for this reaction. A GEANT simulation of DRAGON was used to simulate the ¹³C(p,γ)¹⁴N reaction so that it could be compared to see what fraction of the recoils were being lost within the DRAGON due to this clipping, and also to see where the clipping occurred.

The creation of an ionization chamber in the GEANT simulation for the first time, meant that simulations of the ¹³C(p,γ)¹⁴N reaction could test the DRAGON’s acceptance also, by simulating different mistunes of the DRAGON’s reference tune, in x and y position, x and y angle, and percentage of energy. These mistunes showed that the maximum acceptance for DRAGON is achieved when the beam is not mistuned in x and y position, but mistuned to -0.5% of the energy, and -1.5 mrad and -0.5 mrad in the x and y angular position respectively. They also showed that there is a large acceptance loss, with the maximum acceptance being roughly 78-79%.

aaron_thesis.pdf

Author: Bebington, A. M.

Journal:

An oxygen-neon nova is presently understood to be the result of a thermonuclear runaway on the surface of an oxygen-neon white dwarf. During this event production, and subsequent ejection into the interstellar medium, of the radioisotope ²²Na can ensue. With a half life of 2.6 years, ²²Na β-decays leading to the emission of a characteristic γ-ray of energy 1.28 MeV. This combination of long half life and characteristic gamma signature makes ²²Na a possible “viewing port” into the nuclear physics of these cataclysmic events, for, γ-rays of this energy are readily detectable with past and current orbiting satellite observatories. To date, no 1.28 MeV γ-signal has been observed from any nova, and this remains an outstanding problem in astrophysics.

Within these environments, production of ²²Na can proceed via isolated, narrow reso- nances in the reaction path: ²⁰Ne(p,γ)²¹Na(p,γ)²²Mg(β+νe)²²Na. As many as three res- onant states in the ²²Mg nucleus can contribute to the total nova ²¹Na(p,γ)²²Mg reaction rate. The strengths of these resonances and, therefore, the ²¹Na(p,γ)²²Mg nuclear reaction rate, were hitherto unknown, creating significant uncertainty in the expected yield of ²²Na from an oxygen-neon nova event.

Thick target yield measurements, using a high intensity (> 10⁸ s⁻¹) radioactive beam of ²¹Na with the DRAGON facility at ISAC, have been performed in inverse kinematics resulting in a direct measurement and limit on two astrophysically important resonance strengths ²¹Na(p,γ)²²Mg. Uncertainty in this reaction rate has been reduced by more than 10-fold for nominal peak nova temperatures ≥ 0.3 GK. A narrow resonance, thick target yield curve has been mapped out for the first time using a radioactive heavy ion beam. From this curve, a new mass excess for the ²²Mg nucleus has been derived of −403.5 ± 2.4 keV, rather than the literature value of -396.8 keV. The implications the results of the present work have on nova ²²Na production are consistent with no observed 1.28 MeV γ-signal.

bishop_phd.pdf

Author: Bishop, S.

Journal:

The DRAGON electromagnetic recoil mass separator located at TRIUMF-ISAC in Vancouver, Canada was built to study resonant radiative proton and α-particle capture reactions relevant to nuclear astrophysics. In DRAGON experiments, a stable or radioactive ion beam in the energy range 0.15 MeV/amu to 1.5 MeV/amu from ISAC impinges on a differentially pumped, windowless gas target of hydrogen or helium. Recoiling reaction products are separated electromagnetically from beam ions and detected in the final focal plane, typically with a double sided silicon strip detector. A bench test facility for testing and improvement of the double sided silicon strip detectors and related systems has been assembled. The facility has been used to measure the efficiency and energy resolution of the detectors using an α-particle source, and to assess the effects of radiation damage on the detectors. The pulse height defect and the effect when particles pass through the gap between strips have been measured. Data from α-particle tests have been compared with data from stable beam experiments in the mass range A = 12 to A = 25, and used to predict detector performance in future DRAGON experiments. A 1 MeV/amu ¹⁶O beam has been used to measure timing resolution. DRAGON has used this detector system successfully in measurements of the resonance strengths of several astrophysically relevant resonances in the ²¹Na(p,γ)²²Mg reaction with a radioactive beam of ²¹Na. A hybrid thermoelectric/liquid cooling system has been designed, built and tested with the purpose of cooling the double sided silicon strip detectors and improving their performance. The future possibility of using silicon detectors for particle identification by pulse-shape discrimination has been researched.

wredethesis.pdf

Author: Wrede, C. L. H.

Journal:

The charge state of an ion passing through matter fluctuates as a result of electron capture and loss in the collisions with the target atoms. Despite the existence of a large number of theoretical and experimental studies on this complicated atomic collision system, an accurate prediction of the charge state distribution is still not available. To meet DRAGON’s future experimental needs, the non-equilibrium and equilibrium charge state distributions resulting from the collisions of ¹⁶O, ²³Na, ²⁴Mg ions passing through a windowless hydrogen and helium gas target with the beam energy in the range of 0.138−0.875 MeV/u, 0.200−0.478 MeV/u, 0.200−0.800 MeV/u, respectively, have been measured using the differentially pumped gas target facility at Naples University, Italy, and DRAGON/ISAC, TRIUMF, Canada. It is determined that the equilibrium distribution is established at low target thickness. The equilibrium distribution depends on the projectile species, its energy and the nature of the target, while independent of the incident charge state. The uncertain- ties of the normalized charge state fractions are estimated to be less than 5% except for very low fractions. The equilibrium distribution is shown to be close to the Gaussian distribution. Semi-empirical formulas have been derived for the average equilibrium charge state and distribution width. Assuming that the probability of multiple electron capture and loss in a single collision are negligible, single electron capture and loss cross sections have been estimated using the least-squares method in cases where sufficient experimental charge state fraction data are available. The dependence of cross sections on projectile energy and charge state has been studied.

wenjiethesis.pdf

Author: Liu, W.

Journal:

The "Detector of Recoils And Gammas Of Nuclear reactions", DRAGON, is a new facility, especially designed to measure absolute cross sections of radiative proton- and alpha- capture reactions on radioactive nuclei of astrophysical interest. Located at the TRIUMF-ISAC radioactive ion beams laboratory in Vancouver, Canada, the DRAGON performs studies on reactions in inverse kinematics with ion beams in the mass range of 6 to 30 amu impinging on a gas target at energies of 0.15 to 1.5 MeV/u. A BGO detector array to tag the prompt gamma radiation emitted in a reaction surrounds the target, followed by a recoil mass separator and a double sided silicon strip detector which measures position and energy of the recoil at the final focus. Beam suppression of the order of 1011-1015 is needed to fully separate the readioactive beam ions from the much rarer reaction products. Systematic studies of varioius configurations using stable beams along with measurements of well-know resonance reactions were completed for the commissioning of the complete facility and the energy calibration of the new ISAC radioactive beam accelerator. Additionally, the first results of the scientific program, that has been launched with a study on the ²¹Na(ρ,γ)²²Mg reaction at E_cm ≈ 821 keV, will be presented.

sabine_thesis.pdf

Author: Engel, S.

Journal:

In our every day life we are surrounded by materials composed of the elements of the periodic table. Rarely does one ask where these elements came from. It has been a long process of discovery to understand the precise origin of many of the elements we consider commonplace. It is now believed that the Big-Bang produced only the lightest elements, primarily hydrogen and helium, and that heavier elements were synthesized as the product of nuclear reactions within stars. Occasionally the nuclear reactions that occur within stars produce an isotope of an element which is unstable, radioactive. When a radioactive species decays it emits radiation which is characteristic of the species that decayed. Satellites have been able to detect the characteristic radiation from the decay of several isotopes in the Milky Way. One in particular which has been detected is the isotope of titanium, 44Ti. The decay of 44Ti has been seen in the ashes of exploding stars, vast gas clouds termed supernova remnants. This isotope of titanium eventually decays to a stable isotope of calcium found everywhere on Earth from bones to chalk. It is believed that the bulk of the production in stars of 44Ti occurs as the star explodes, during the supernova. Calculations indicate that among the many possible reactions during a supernova, a particular nuclear reaction, where calcium captures a helium nucleus and fuses into titanium, is the main source of 44Ti. In this work it is detailed how using laboratory equipment on Earth one is able to shed light on the nuclear physics of this particular reaction governing the production of an isotope in our universe.

theses-10-07-CvOthesis.pdf

Author: Christian Ouellet

Journal: